psql \dt and table size

It stroke me today again, that \dt+ isn't displaying the acurate table size
for tables, since it uses pg_relation_size() till now. With having
pg_table_size() since PostgreSQL 9.0 available, i believe it would be more
useful to have the total acquired storage displayed, including implicit
objects (the mentioned case where it was not very useful atm was a table
with a big TOAST table).

Attached minor patch extends \dt to use pg_table_size() starting with
PostgreSQL 9.0, not sure if we backport such changes though. It would be
interesting for 9.1, however.

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Thanks

Bernd